Clean sound from a Peavey Windsor?

[jim p]

I was told by someone that they could not get any clean sounds from a Peavey Windsor and thought that can’t be right must just be settings. So interested I found schematics for the amp on the web and looking at all the preamp gain I have become a believer. I suggested back to them to remove at least two of the bypass caps C27 and C5 then change R55 to 50k or 100k. Also the decoupling capacitor C28 may need to be removed. Then for more headroom on the first two stages you could change R38 in the power supply to a 10k. What could be a nice fix is if a push pull pot could be used to replace the preamp volume pot so you could wire in some n-channel mosfets to put the cathode bypass caps back in for a high gain setting like stock.

Anyway has anyone out there modified the preamp in one of these amps and how did things turn out.

[tunghaichuan]

It looks like the gain is padded way down already from the two voltage dividers. Maybe lower the value of R6 from 2.7k to 1k or 820R or even 680R?

Maybe substitute a 5751 or a 12AY7 for the first preamp tube?

That's all I can think of.

tung




I was told by someone that they could not get any clean sounds from a Peavey Windsor and thought that can’t be right must just be settings. So interested I found schematics for the amp on the web and looking at all the preamp gain I have become a believer. I suggested back to them to remove at least two of the bypass caps C27 and C5 then change R55 to 50k or 100k. Also the decoupling capacitor C28 may need to be removed. Then for more headroom on the first two stages you could change R38 in the power supply to a 10k. What could be a nice fix is if a push pull pot could be used to replace the preamp volume pot so you could wire in some n-channel mosfets to put the cathode bypass caps back in for a high gain setting like stock.

Anyway has anyone out there modified the preamp in one of these amps and how did things turn out.

[bigG]

From Peavey's literature and various reviews, the Windsor Studio was not built for clean sound, but specifically for a Marshall-like gain, which is pretty much what it has, in spades!

If you want gain, it's pretty nice. If you want clean, this ain't for you. :)

[SharkUSMC]

I put a really nice 1960 Westinghouse 6201 12AT7 Reverb Driver in my first preamp tube slot and it went a very long way to taming the beast. It definitely altered the sound and tone of the amp more than I was expecting. I can get a clean tone up at ear splitting volume now, whereas before it was nearly impossible.

Set the guitar volume around 5 (squier classic vibe strat w/ weak p'ups) preamp gain around .5, and I can crank up the master louder than I want to hear it and still be clean. I only run the amp in A/B mode, I don't fool w/ that knob or the presence/resonance knobs at all.

Prior to the tube swap I couldn't stay clean with the volume anything other than extremely low.

[jim p]

Thanks for the feedback it is good to know a low gain tube such as a 12AT7 in place of V1 worked. I was going to suggest to him to change V1 and or V2 to either a 12AT7 or 12AY7 to see if that would lower the gain enough to get a clean sound. The mod has not been done yet so I suggested just removing C27 and C5 before making any other changes. Removing C27 will reduce the gain of the first triode to 20 from approx 52. Then removing C5 will change the gain of the second triode from approx 55 to approx 33 so equal to or better then going to a low gain tube. As I stated before it would be nice to add a switch to pull the bypass caps in and out for a high gain/low gain switch. If you can change the preamp gain pot to one with a pull switch same as the guitar mod for a humbucker the amp will look stock. The other way to go would be to use one of the input ¼ inch jacks if you do not use the low gain input for a switch location.

For what it is worth this is the head I am talking about not the combo.

[SharkUSMC]

If it is any consolation I thought it was just a settings thing too until I had a friend with decades of amp experience fiddle with it and he couldn't get a clean out of it either.

I'm real happy with the swap, and I still have the ability to get that dirty sound if/when I want it (rarely if ever) and crank out enough volume to kill small animals.

[jim p]

I have been going over the schematic of the pre amp section on this head amp all week figuring out the over all gain by calculation, load line and using spice simulation and figure you really need to cut the gain to get a completely clean sound through it. So you will need to remove the cathode bypass capacitors on the three triodes plus change the value of R55 to around 50k ohms to knock it down. Also the value of R4 and R5 may need to be changed to 1.8k to 2.2k ohms for a bit more gain reduction. The chain also has a high amount of brightness from C37 and C36 (good for overdrive but not clean) so you need to either remove C36 or parallel R55 that is changed to 50k with a 4.7nf capacitor. I have attached a schematic that shows all the changes. I have not done the mod myself but should know the results from someone who is making the mod this weekend.

I am also attaching a schematic that will allow you to switch between the stock pre amp chain to the clean chain using n-channel mosfets to pull the components in and out of circuit. I did not show the actual switch connections what you will need is a voltage divider connected to the +24 volts that will pull the gates of the mosfets high figure 50k ohms connected to ground then to the gates with 100k from the gates to +24 so the gates will be at 8 volts for on. Connect the switch to ground the gates to turn them off. You can also add a capacitor to the gate connection to debounce the switch if you want. For the mosfets the BS270 which cost 18 cents each at Mouser electronics should do fine. So for clean you will turn off Q1, Q2 and Q4 while turning on Q3 then for overdrive you turn on Q1, Q2 and Q4 while turning off Q3

Let you know how the mod goes.

[ibanezjunkie]

personally id just roll the guitar volume back and replace the preamp tubes with something lower gain, maybe 12AY7s.

[jim p]

Even with a low gain tube such as a 12AT7 or 12AY7 there will probably be too much gain in the pre amp chain and too much brightness due to the caps in parallel with the 470k resistors. Plus if you go to the full mod using the mosfets you can have both clean and overgain.

[jim p]

These heads are now $249 at MF not bad price for a 100 watt tube head even if it needs some mods to get it where you want it.

I just bought a used one from MF for 150 bucks with shipping that can not be returned. Hope it has all the tubes in it, even if it doesn’t I guess all the other bits will still add up to be worth the price.

To add to this I think I have it worked out where I can mod the amp to have a set of EL34 tubes and a set of 6L6 tubes add a switch and use the amp for 50 watts out. So the output will be either two EL34s or two 6L6 tubes. Loose the class A to class A/B pot (unbalanced phase splitter?) and use it for reverb level and add reverb. Add the switchable clean or overdrive described previously, so should be fun as long as what I bought is not a box of rocks.

Well the amp has left Earth, well Earth City and is on its way. Since then MF has put the price back up to $299 for a new amp (but shipping is free). Also have been reading that an unbalanced push pull output may result in a desirable tone.

[jim p]

Well I got my amp from Musicians Friend today in a beat up box packed with a 220 volt power cord. Looking at the amplifier and schematics I could see that the amplifier was wired up for 110 volts so figured good chance something is going to be fried in the amp. You could tell by the plastic top that the fuse holder had been opened and closed a few times but the fuse was good. So took the amp apart took out all the tubes and started to have a look around. The internal fuses were fine so that was a good sign. With the power switch on using an ohmmeter across the line input it read as an open so maybe the primary of the power transformer is shot, also the transformer label says it has thermal protection so? Then checking the primary directly I had a short so maybe the transformer is ok. Then I took out the PC board the IEC line input and fuse are mounted on and found a bad solder connection on the line input resoldered it and dam ohmmeter readings were good. So fired it up with no tubes and a light bulb in series to play it safe and all the power supplys are good. Have the tubes in signal generator hooked up power resistor on the output for a load a scope hooked up and the amp works fine. So on to checking things out and then modifying this thing. Not bad for 140 bucks.

[bigG]

Man, I envy you tech savvy guys! Way over my head.

But, on the other hand, I'm kinda glad I don't break a guitar or amp down to the basics of what makes any particualr one play and sound the way it does. To me, that stuff is "magic", and I'd rather just plug in and enjoy the magic w/o all the "nuts n bolts" understanding. Guitars and amps are works of art to me, is I guess what I'm saying. I like to just appreciate the entire piece for what it is and does rather than get into the why's and wherefores...

Whatever floats your boat, dudes. I do envy your knowledge! And, w/o people like you, these pieces of magic that I love wouldn't exist!

G

[jim p]

I have started to mod the amplifier I used a scope and checked out the preamp and figure there is just way too much gain in the preamp on this amp. One of the first things you should do if you are going to work on this amplifier is to add a bleeder resistor on the power supply. I had the amplifier off for 20 minutes and stated to take the locktite goop off the screws to take the PC board out then stopped to check the power supply and it had over 300 volts on the caps. So blow a few cents and add a 470k 1 to 2 watt resistor in the power supply section to bleed down the caps when the amp is off I guess Peavey has a low regard for service techs and factory workers. So this is your basic cheep single sided no plated holes PC board so you will need to pull the board out to change any parts on the board. In order, first I reduced the gain of the input stage triode from stock gain of 60 (-3db at 90Hz) by removing C27 for a gain of 21. On the next stage change the stock gain of 55 to 22 by removing C5 and changing R4 to 2.2 k ohms. On to the third stage stock gain of 58 changed to 22 by removing C28 and changing R5 to 2.2k. Now the amplifier is clean with preamp volume up to 5 (WOW this goes to 12 not just 10) and still have plenty of over drive at 9 to 12. I have been using the amp with just two output tubes installed (removed the middle two) for 50 watts instead of 100 watts but the taps on the transformer are double the value this way so 4 ohm tap to 8 ohm speaker, 8 ohm tap to 16 ohm speaker and 16 ohm tap to 32 ohm speaker when you do this.

Options on the gain changing with the above values you can go back to maximum gain on any of the stages by bypassing the cathode resistor with 1uF or greater. Also you can reduce the gain less by changing the cathode resistor to a lower value. An example If R6 of the first stage is 1.5 k ohms with no bypass the gain of the tube will be 30 instead of 21. You can get to that value by paralleling the values in the PC board with the board installed so you will not have to take the PC board in and out. You can also add the bypass cap across the resistor with the board installed so once you make the mods above you can play with other values to see if you like them better.

I have not done anything with all the brightness in the preamp stages from C37 and C36 it is a little lower without C28 (-3db at 230 Hz) I might change C41 that is in the amp to start killing all signal starting at 1.6 kHz. Maybe a resistor in series with C41 or a lower value say 500pF.

I find the resonance control has little effect so it may become my reverb level in the future. The class A to class AB has little effect also and is just a control to unbalance the phase splitter so not really class A.
On the parts front all the 12AX7s are JJs the output tubes Ruby (China?) EL34s the transformers are made in Taiwan. There is plenty of room to grow in this amp I guess bigger is better?

So may stop here and get others opinions on the amp then add two 6L6 tubes the bias and support shrubbery and mosfet switches on cathode resistors so can switch between EL34 or 6L6 for 50 watts and maybe run all four for 100 watts. Then get a reverb tank in the amp with solid state drive and recovery.

[jim p]

Well since yesterdays mod have been surfing the web and ran across the schematic for the JCM800 which looks to be the amp of choice to copy. The mods I did had the amp with close to the same over all gain and brightness as the preamp section of that amplifier. One of the differences that I decided to add is to change V2B to a cathode follower to feed the tone stack. The stock setup only had a gain of two so just changed R5 back to 1k to recover the gain loss from the change.
On to the mod if R5 is stock you are good, if not change to stock value of 1K. No capacitor installed for C28.
Remove R70
Remove R56 replace with 10k
Remove C18 replace with a jumper
Remove R65 replace with 10k (can use this 100k for value change of R21)
Remove C41
Remove R21 replace with 100k 1 watt
Remove C6 connection to the plate of V2B connect to cathode of V2B

From calculation the overall gain at 800Hz of the JCM800 is approx 6740

The over all gain with yesterdays mod combined with this mod is 7030

The input of this amplifier is the Fender style high impedance low impedance set up which is not the same as the Marshall high gain low gain setup (low gain bypasses the first stage triode) of the JCM800 so I may figure how to cut and jumper the ¼ jacks to do the Marshall style set up.

One note on this mod the only thing that might cause a problem is the cathode voltage for the cathode follower is close to the maximum allowable voltage difference between the heater and cathode but the stock JJ 12AX7 seem to be fine.

[kiteman]

I was told by someone that they could not get any clean sounds from a Peavey Windsor and thought that can’t be right must just be settings. So interested I found schematics for the amp on the web and looking at all the preamp gain I have become a believer. I suggested back to them to remove at least two of the bypass caps C27 and C5 then change R55 to 50k or 100k. Also the decoupling capacitor C28 may need to be removed. Then for more headroom on the first two stages you could change R38 in the power supply to a 10k. What could be a nice fix is if a push pull pot could be used to replace the preamp volume pot so you could wire in some n-channel mosfets to put the cathode bypass caps back in for a high gain setting like stock.

Anyway has anyone out there modified the preamp in one of these amps and how did things turn out.

How much improvement is there by just lifting the caps? I mean by unsoldering one ends of the caps and leave 'em there.

EDIT: this way it's reversible.

[jim p]

If you just lift the bypass caps the gain with the preamp volume at maximum at V2A will be approx 14,000 at around 800Hz. While in the mod with the cathode resistors increased the gain at that tube is approx 5600 so it will still be a lot of gain with just no bypass caps. If you increase the value of the cathode resistors and then put back in the bypass caps the gain of the tube will be the same as stock even with the new resistor value (the operating point of the tube will be different though). As soon as you put a bypass cap of a large enough value across a cathode resistor then the gain is determined by the tube and its load. So if you put a 2.2 uf across any of the unbypassed cathode resistors the gain of that tube will be approx 55 to 60 same as stock.
As far as lifting one side with this being a single sided board you cannot do a good job of soldering and resoldering from the top of the board as you look in the amp. So if you take a cap out it may pay to remove it completely so if you want you can solder it across the resistor if you want to put it back without removing the board.
I can not say how the amp is with just the bypass caps removed because I never tried it.
As I stated before there are no bleeder resistors in this amp so use a volt meter and be careful.
Hope this gives you an idea of what you want to do.

[kiteman]

Thanks, I learned enough to leave it alone. :whatever:

[jim p]

Decided to add the same high gain low gain input set up as the JCM800 to the Windsor. To do this two traces will need to be cut on the bottom of the PCB and two jumpers installed. One resistor R2 will need to be removed also. You can see these in the attached pictures and the marked up section of schematic.

Another modification I have added is a 10pf cap from plate (pin 1) to control grid (pin 2) on V2A this will limit the amplification of high frequencies to roll things off a bit. The value of the cap added along with the tubes plate to grid capacitance (1.7 pF) is multiplied by the gain of the tube stage so 11.7 pF looks like 38 times 11.7 pF for the same as having 445pF to ground installed (this is the Miller effect). Another way to do this would be to increase the value of R14 to say 68k to 100k instead of the stock value of 10k. This will cut some high frequency buzz you may be hearing from the amp. I have also found I prefer the sound of the amplifier with out C37 installed so I have it removed.

Well since the mods above in checking the amp out decided that a bit more gain maybe better relative to having a guitar that’s pickups are not on the hot side. Plus you can always plug into the low gain input bypassing the first stage for no overdrive. I also decided to add some brightness back in at C37 but put 100k ohms in series with the 470 pF cap to limit the maximum level. For the gain increase I changed R6 to 1.5k so with no bypass cap the gain of the first stage is approx 30. Then on the second stage changed R4 to 1.5k with no bypass so its gain should also be approx 30 also.

[jim p]

I am moving on to checking out the power amp section of the amplifier using a scope and signal generator. First off wanted to see how much the Class A / Class A/B control has and it does distort the output signal quite a bit so maybe it can be useful. As I stated before with out changing the grid bias on the tubes it cannot be a true Class A operation of the output section. Next wanted to see how much feedback was in the amp and what level of control there is. I just unplugged P400 to see how much gain there was between the control grid of V3 A at pin 2 to R47 and had approx 200:1 With the feedback connected and presence dialed out gain is approx 95:1 As the amp is stock you cannot dial the feedback out with the presence control until the frequency is above the -3 db point of approx 300Hz. I decided to make it so the feedback can be totally dialed out so changed C9 to 10 uF cap a 1uF cap would probably be low enough, but it is what I had around. I also added a greater level of feedback by changing R47 to a 15k ohm from the stock 33 k ohm. I don’t see much effect from the resonance control so I decided to bypass it by jumpering around it and making C9 a jumper. I will use this pot as the reverb level control when I add reverb to the amp may try to get a lower value pot for this 10k log would be best. Another thing I found with feedback and high signal levels the output section has an oscillation when running Class AB that stops with the texture pot dialed just slightly towards Class A. Found this to be equal to 15k in series with the signal to the control grid of the output tube. In the future I plan to add a 25k resistor in series with C15 to see if this works well for a final solution for the oscillation.

Started to add cathode resistors to the amplifier for setting up the tube bias and also the ability to shut down the tubes using mosfet switches. The stock amp is not nice enough to have stock low value resistors in the cathodes of the power tubes for setting up the amp output tube bias. With all the jumpers they needed on the output section PCB it would probably have been easy to design in some cathode resistors. I just did the mod to two tubes adding 10 ohm 1 watt resistors also added 5 k ohm 3 watt resistors between the cathode and ground. With the 5 k resistors in the cathode connection to the tube I had a maximum signal out of the amp equal to 0.1 watts so looks good just need to wire in the mosfets across the 5 k resistors for the ability to gate the tubes on and off for the EL34 / 6L6 combination.

Added a pic with notes on mods done to the preamp section

[jim p]

It works! I now have the amp set up to run either a set of EL34s or a set of 6L6 output tubes. Not sure if I am getting a fair idea of the tonal difference between the tube sets because I think the set of used 6L6 tubes are down on power at an idle bias of 38mA.Had to run the bias up to 50mA to get a match to the EL34 for output power. I will post a set of marked up schematics and pictures later. The BS270 N-mosfets work great to switch the tubes in and out and can just use the rectified heater supply to gate them on and off. The grid bias supply is a bit of a hassle the way it is done and I will need to change it to supply more current for the reverb circuit when I add it. I may adjust the signal input level to the tubes for 100 watts with both sets running will need to half the signal to the EL34 to match the gain (transconductance) of the 6L6 for that. But so far so good.

Been messing with the amp a bit and you can switch from tube set to tube set with the amp running, you don’t even hear a thing while switching. The gain difference between the EL34 and the 6L6 is not as bad as I thought it would be maybe 20% less for the 6L6. Had to run the bias hot on the 6L6 close to 50mA at idle. A new set of 6L6 tubes maybe the ticket to even things out.

[jim p]

I am attaching a marked up PDF of the output section of the schematic as well as a schematic that is just the circuit for each cathode. The circuit is a 10 ohm resistor to use to set the tube bias, if you want you can use a 1 ohm resistor use 1 watt or greater for these resistors. The cathode impedance of the EL34 is 90 ohms so 10 ohms will lower the gain but not much. While the cathode impedance for the 6L6 is 190 ohms so 10 ohms will matter even less. There is a 5 k resistor connected to ground with a BS270 mosfet across it connected to the 10 ohm resistor connected to the cathode. With the BS270 turned off the current through the 5k almost completely shuts down the tube I found at best I could get 0.1 watts out of the tube. When you turn on the BS270 its on resistance is from 1 to 4 ohms so the tube is completely turned on. The maximum threshold voltage for the BS270 is 2.5 volts with a transcondunctance of 330mA/volt so the rectified dc heater supply works fine to use as the voltage to turn the mosfet on. So I connected the two mosfets attached to the cathode resistors of the EL34 tube set to one side of a double throw switch and the other two mosfets connected the 6L6 tube set to the other. Then connected the dc heater supply to the wiper of the switch. You can switch back and forth between the tube sets with the amp running no pops or drop in sound level.

The bias circuit in the amp is interesting the input is -108 volts peak the value of R 20 limits the current available to charge C1 and C4 and supply the load of VR9 and R23. As a wet finger guess it looks like a divider with -108 volts in but the value of R20 is a factor of 4 times its resistance for calculation. So in my case I paralleled VR9 with a 5 k pot for the bias circuit needed for the 6L6 changed R20 to 2k and R23 to 5k. The 5 k pot in parallel with the 10 k pot is equal to 3.33k. As a divider R20 2 k ohms times 4 is 8k the two pots equal to 3.33k plus 5 k for 8.33k so the top of the pot looks like 8k in series with 8 k for ½ of -108 volts for approx -50 volts. With this I have approx -51 volts to -30 volts across both bias adjustment pots. C1 and C4 are 63 volt caps so it is important to keep a meter on them on power up if you make a change to make sure you do not exceed the voltage rating. Also maybe you want to start with a value for R20 greater then calculated to play it safe. Down the road I will need to change this circuit to use a zener diode and series dropping resistor to allow me to use this as the raw supply for the op amp drive and recovery circuit for the reverb tank. Also the way this bias supply is if the line voltage is low the bias voltage will go positive and the tube current will rise. This will be offset by the lower heater supply and screen grid voltage but I doubt these are going to balance out. This is something to keep in mind if you run the amp line voltage down with an autotransformer you may be running the output tubes hot (high current) which can shorten there life and also may damage the output transformer.

You will need to break the circuit to the control grids of the 6L6 tube set free from the inputs to the EL34 tube set because of the different bias voltage required. I capacitive coupled the signal to the 6L6 on the circuit board and installed 100k grid resistors to the new bias supply.

One thing I noticed the way the amp is set up the suppressor grids of the EL34 tubes are connected to the bias supply instead of ground. Not sure what difference this will have on tone but one thing you might want to try is connecting them to ground instead of the bias supply. As far as the 6L6 they have no suppressor grids so it doesn’t matter to them.

Have looked around and see at “Tales from the Tone Lounge” Marshall myths he is recommending to connect the suppressor grids to the negative bias supply. From what he states the tube is more linear and the gain is increased. While you are there he has mods for the JCM800 which can be applied to this amp as well so you may want to check them out.

[leadfootdriver]

:confused: I'm trying to follow along. How did u learn so much about electronics?

[jim p]

:confused: I'm trying to follow along. How did u learn so much about electronics?
Electronics is my 9 to 5 job doing analog design. As far as tubes I have a few books but a lot of the tube theory can be found on the web. Problem is some of the information is bad and you have to have some idea of what you are looking at. If you look around there is basic electronic college course information on the web so you can use that to learn more. One great free thing to use is LTspice that Linear Electronics has as a free download. This is circuit simulation software that will help even to learn basic electronics.

[jim p]

Being I have added the mosfet circuit to the cathodes of the output tubes it occurred to me a bias voltage protection circuit could be added to the amplifier. If the amplifier for some reason lost the grid bias supply to the output tubes the grids would go to zero volts and the tube would be turned on completely. To prevent this a simple op amp comparator circuit that is connected to the bias supply and used as the positive voltage to gate the cathode mosfet switches will shut the tubes off if there is no negative grid voltage. This circuit could be added to any grid bias amplifier as protection for the output tubes and output transformer.

The value of R1 and R2 depend on the positive supply voltage and the negative bias supply voltage. When the bias supply is at its operating voltage the input at the negative input of the op amp from the divider should be just negative of zero volts. The positive supply needs to be a stable voltage with some regulation a zener or regulator IC supply. R3 is an option to add hystersis so the comparator will not oscillate changing the threshold voltage of the comparator relative to its output voltage. The diode in the schematic is an option the op amp used may require for protection. Good chance a low voltage op amp could be used that could be supplied by the DC heater supply with ground as its negative rail.

If the bias supply is not present or does not go negative enough the output of the op amp will be at its negative rail so the mosfets can not be turned on. If the bias supply goes negative enough the op amp output swings positive and the mosfets can be turned on. Also if the bias supply fails during operation the mosfets will be turned off and the output tubes gated off.

Another circuit instead of the resistor divider would be a diode in series with a zener diode used to regulate the negative grid bias supply. So with no current flow through the zener the output tubes would be shut down. You will also need a pull up resistor connected to the cathode of the zener and anode of the diode to the negative input of the op amp.

[leadfootdriver]

Electronics is my 9 to 5 job doing analog design. As far as tubes I have a few books but a lot of the tube theory can be found on the web. Problem is some of the information is bad and you have to have some idea of what you are looking at. If you look around there is basic electronic college course information on the web so you can use that to learn more. One great free thing to use is LTspice that Linear Electronics has as a free download. This is circuit simulation software that will help even to learn basic electronics.


:rockon: Awesome. YOu're my goto guy now! :D

Thanks for the website tip! :beer:

[denni two tone]

im new here, but been playin guitar for 40 plus years and im a rocker,ive had amps from grande,sunn,fender marshall the list goes on,the windsor amp is a jcm 800 on steroids, i just bought one because it was so cheap priced, i love the gain on this amp and the tone for any kind of rock,i found this forum searching for ways to clean this amp up,long storie short, i change v1 valve to a gt 5751m wow what a change, i can actualy back the volume pots down on my les pauls and it cleans up nice,not fender nice ! but crisp clean sound,added reverb thru the the effects loop, and it even sounds fenderish, im playin mostly gibson les pauls and a few exploers, so i think you strat tele guys would clean up a bit better, and the high gain hiss disapeard, try it youl like it without a big mod, let me know what you think?:rockon:

[Bitzy]

Electronics is my 9 to 5 job doing analog design. As far as tubes I have a few books but a lot of the tube theory can be found on the web. Problem is some of the information is bad and you have to have some idea of what you are looking at. If you look around there is basic electronic college course information on the web so you can use that to learn more. One great free thing to use is LTspice that Linear Electronics has as a free download. This is circuit simulation software that will help even to learn basic electronics.

Jim P, is it possible for you to the schematics for the preamp and power amp noting all the changes you have made so far?

I would also be interested to know if you have added the reverb and how it went.:dude:

[jim p]

If I get a chance I will look back through what I have posted but I think there are schematic mark ups and some pictures of what I have done so far. I have not gotten to the reverb yet, as far as drive and recovery circuits and reverb tank they will be the same as what I used on the Crate V5. So for schematics on that you can check out the postings at “Crate V5 mods…anyone? “. What will have to be done is to change the negative supply used for the grid bias so it can also supply the reverb circuits also. Should get to the reverb soon I have been repairing an old Carvin amp and trying to design a snap action over current shutdown circuit for the plate supply on any amplifier.

One thing they have at Carvin is a step by step on how to bias an amp that just measures the total supply current to check the bias. I don’t think this is the best way to set the bias but if you can’t measure the cathode current on each tube it gives you something.

[jim p]

I haven’t made any changes in this regard but if you go to the “Crate V5 mods…anyone” thread I posted an article under “tube bias and tone” that may make you want to consider different cathode resistor values for R4 and R5. At the minimum you may want to play with the value of R5 say change it to 7.5k to 10k for more of a Rock crunch. Or change to a value of 500 to 750 ohms for a Blues type of distortion. Also you can add the circuit I posted to switch between modes for the most flexibility.

[StringsOfJoy]

I've been running 4 x KT88s on the Windsor with around 65-70% idle plate dissipation just using the stock bias circuit and I like how that sounds better than EL34s or 6L6GCs. Been doing it for a few months with light play and I've experienced no problems or significant bias drift.

I'd love to have to option to run them cooler than this, but the stock bias pot only allows me to sweep down to that 65-70% range.

Am I right in thinking that inserting another resistor (maybe a pot) in series with R23 would give me the option of running the KT88s cooler?

From what I've read, I won't get optimal performance from the KT88s unless I can get also the screen grids to 220k, but for now I'm more concerned about getting the bias circuit to play well with the new tubes and can think about the screen grids later.

Since the amp does not need to reproduce bass guitar frequencies, I'm not concerned about swapping trannies. I just want a fairly quick and least invasive method of running the KT88s.

[jim p]

Not sure which way you want to go I would think you want to take the bias voltage more negative. One thing you need to worry about is the voltage rating of the two filter caps C1 and C4 at 63 volts. If you reduce the value of R20 this will drop the voltage lower on the bias adjust circuit, I posted previously that R20 looks like four times its value in series with the pot and R23. The voltage at the diode is 77 VAC or 109 peak so a slight reduction of the value R20 will take your bias supply more negative.
Not sure how you are biasing the amplifier if you are using a scope and a waveform generator then you can look for crossover distortion. If you are just going by current and voltage on the tube then 70% to 80% of maximum plate dissipation is where you are supposed to be.
Check this page http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf
And this one
http://www.tone-lizard.com/Biasing.htm

Just to add to this I just got done re-tubeing a Carvin amp with four 6L6WXT Sovetek tubes and the total supply current is 150mA. I went by crossover distortion using a scope and waveform generator with the input signal at the diff amp splitter. So plate dissipation on this amp is only about 50% at idle.

On the KT88 one thing I see on the data sheets is it calls for a control grid resistor or 100k at what looks to be when plate and screen grid power dissipation is above 35 watts. That would mean that R10 and R9 in the amplifier would need to be 50k ohms which is a heavy load for the phase splitter. Even if you stay below 35 watts looks like R10 and R9 should be 100k instead of 220k to prevent thermal runaway of the tube.

[StringsOfJoy]

...and thank you for responding to my question...this is the only place I've seen online that has mentioned bias mods to the Windsor in serious detail, so I appreciate your sharing what you know.

I'm not using a scope and not using the crossover distortion method to bias. I don't have a scope or signal generator, and based on my reading so far I feel more comfortable using the current and voltage method using a multimeter and a Weber Bias Rite adapter: I care less about eliminating crossover distortion than I do about keeping the tubes within the relatively "safe" arbitrary zone under 70% idle plate dissipation. In fact, I want the option of going well into the 30% range (well into the crossover distortion zone and roughly where Peavey sets its stock bias) to preserve tube life even though I generally prefer biasing in the 60-70% range.

I was lucky enough to find a quad of Penta KT88s that allow me to bias just under 70% idle plate dissipation, but I have another pair that I cannot plug into the amp without exceeding the 70% "magic number". Certainly biasing into the 50% or 35% range would be out of the question with those tubes or the ones I have in right now. So that's the main interest that spurred my question.

The problem is that I'm not sufficiently experienced at understanding the circuit and the math behind it to understand which resistor values in it would have to change in order to allow me to bias ANY 4 well-running KT88, say, anywhere between 35% to 70% idle plate dissipation. I do suspect, however, that there may be more than one way to skin this particular cat.

When I was first trying to bias my amp close to 70% idle plate dissipation, I was using the amp's original Ruby EL34 BSTRs. It was impossible to get much higher than 55% with VR9 swept to offer minimal resistance.

Using alligator clips, I connected a 250k sweepable Honeywell pot in parallel with R20, at first, then with R23. I began with the maximum resistance set on the pot, close to 250k, which I thought would have the least impact on the original resistance at the corresponding resistor.

As I swept the pot toward its lower resistance settings, I noticed that effectively lowering the resistance on R20 had a minimal effect and it was difficult to find a precise relationship between sweeping the pot from max to lower resistances.

On the other hand, connecting the pot in parallel with R23 showed a direct and significant effect on the idle plate dissipation I was measuring at the tubes when I swept the pot. The negative voltage increased as I lowered the resistance of R23 by sweeping the 250k pot toward it's lower resistance settings (the tubes ran hotter), and it was clear that a parallel connection with R23 would easily allow me to run my EL34 BSTRs at a desirable 60-70% range.

It's really mainly because of this little experiment and measurements that I thought to ask first about changing the resistance value at R23 instead of R20. I reasoned that, with the KT88s in there, if I INCREASED resistance (which I couldn't do...as far as I know...by connecting anything in parallel with R23) by connecting another resistor (maybe a variable one in the form of a pot) in SERIES with R23, that would increase the resistance offered by VR9 and R23 (and the new resistor/pot) and thereby decrease the negative voltage and current flowing into the tubes. Is this correct?

If it is, what would be the math I would need to use to determine the value of the additional resistor to place in series with R23 (and VR9)? It seems like I could replace R23 with a pot or a pot and a fixed value resistor (of lower than 39k resistance) and be able to run either KT88s or EL34s in the amp at the desired range.

If I'm not correct and R20 is really the better focus for the mod, I'd like to understand why it works and how the voltage ratings of C1 and C4 being limited to 63 V fits into the equation.

Also on a side note, please let me apologize for a a confusing mistake I made above. Instead of saying that 220k for R10 and R9 would be required to use KT88s optimally (ie., squeezing the max power dissipation from them), what I meant to say was that the spec sheets seemed to call for 100k when one is running them in a circuit where KT88s will be made to dissipate more than 35 Watts max.

That's my understanding of what the spec sheets mean when they call for 100k resistors there (at R9 and R10 in the Windsor circuit). In fact, I thought I understood that it was the fact of lowering the resistance from 220k to 100k at R9 and R10 that would "open up" the tubes to potentially run at the ~42 Watt per tube max plate dissipation (and that further assumes you have the trannies to provide enough juice to the tubes). Am I right about this?

Thank you for your indulgence. As you can tell, I'm not very experienced at understanding the mathematics or design of bias circuits and specifically the one used in the Windsor. That may explain a bit if my questions seem confused or not quite correctly put. Still, I am trying to learn and try something new that I think is helping me achieve my own happy tone through this amp. I'm hoping that learning about this bit can help me understand amps better and better over time without being afraid of trying and sharing the little bits I learn.

Thank you again. I'd appreciate any bit of wisdom you can impart related to the stuff I'm asking about and I am really grateful for your response posted above!

[jim p]

Just want to make sure we are on the same page relative to the cathode bias voltage on the output tubes. If you want to reduce the idle plate dissipation of an output tube you would take the bias voltage more negative say if it was -40 volts to reduce the power dissipation you would go to -45 volts as an example.
On the stock bias circuit in the Windsor the AC secondary voltage is 77 VAC equal to -109 volts peak because the filter capacitors are after R20 this reduces the DC voltage on the filter caps C1 and C4. A quick approximation of the voltage on C1 and C4 by calculation is to multiply the value of R20 by 4 then R23 and VR9 are there actual value and calculate the voltage as a voltage divider with -109 volts applied. So in the stock circuit you have 60k ohms (4 X 15k) in series with 10k plus 39k ohms total of 109k ohms. So the voltage at the top of the pot is 10k + 39K / 109k times -109 volts for -49 volts approx as the most negative voltage you can set the pot too. If you change the bias voltage circuit the most negative voltage with the stock capacitors you could have would be approx -58 with out possible shorting of the filter capacitors (they may over heat and explode if voltage rating is exceeded).

On the subject of grid resistors on tubes with power tubes you will see that there are usually two values one for fixed bias and one for cathode bias. What happens in the tube is that sometimes instead of just electrons flowing an electron with sufficient velocity hits a molecule of stray gas in the tube or impacts the plate with enough velocity to produce a positive ion. The positive ion being a positive charged particle will head for the most negative element in the tube this can be either the suppressor grid in a pentode or the control grid. When it is the control grid it will make the control grid more positive relative to the voltage drop produced across the grid resistor and increase the plate current in the tube. If too high a value of grid resistor is used the tube can go into thermal runaway where with higher plate current more ions are produced increasing plate current increasing ions, increasing plate current and cherry red plate in the tube with the possible demise of the output transformer and/or tube. When cathode biased the cathode resistor voltage will increase with plate current and give negative feedback to help prevent this thus the higher value of grid resistor that can be used with cathode bias versus fixed bias.

Why the KT88 needs such low values of grid resistance when driven hard may be due to ions produced from the velocity of the electrons impacting the plate. Also the KT88 is a power beam tube which is a tetrode with beam forming plates the same as a 6V6 or a 6L6 so there is no suppressor grid that might reduce the unwanted control grid current.

Relative to the values of R10 and R9 due to the fact they are connected to two tubes the data sheet value of maximum grid resistance will need to be divided by two. So if the data sheet called for 500k ohms you would need to use a maximum value of 250k ohms.

The value of control grid resistor used has no effect on the power capability of the tube it is attached to. Lower values will be more of a load on the preceding stage that is driving the tube. Also lower values will effect bandwidth and reduce possible blocking distortion.

[jim p]

I went with a solid state reverb drive and recovery amplifier set up for adding reverb to the amplifier I have. The reverb tank I used is the 8BB2A1B that I bought from Antique Radio Supply same as when I added reverb to the Crate V5 here http://www.thefret.net/showthread.php?t=9379&page=11
It would be best to go through the mod on the V5 posted above to see what to do relative to building and testing the reverb drive and recovery circuits. I have made a change to the recovery amplifier it looks like the best op amp to use at a reasonable cost is the NE5532. Also to reduce noise I am using both op amps in the dual package to make the recovery amplifier a differential amplifier to help lower noise pickup from the tank to the recovery amplifier. So another change that needs to be made will be to isolate the RCA jack on the output of the reverb tank as well as the input of the reverb tank. Also a ground wire will need to be added from the reverb tank to the chassis of the amplifier. Along with this if you are into making up your own cables would be to use a twisted pair inside of a shielded cable for the reverb tank output. The twisted pair should be connected to the inner and outer connectors of the RCA jack on the reverb tank while the outer shield would be connected to ground near the recovery amplifier the shield should be left floating on the reverb tank side.

To supply the op amps I made modifications to the bias circuit in the amplifier. So before you start you should measure the bias voltage on the output tubes to know where to set it to after the circuit modifications. R20 was changed to a 500 ohm 3 watt resistor and a series string of three 18 volt 1 watt zener diodes added to create a -18 volt supply for the negative rail of the op amps. For the positive supply the stock circuit has two 1N4744 15 volt zener diodes so you have +15 volts across one of them in the amplifier for the positive rail.

To sum the reverb signal into the output I added 220k ohms in series with the control grid of V3. Then the output of the recovery amplifier is connected to a 100nf capacitor and a 10k pot with the wiper of the pot attached to a 330k ohm resistor that is connected to the control grid of V3. To prevent feedback oscillation of the reverb the value of C35 was increased to 200pf from the stock value of 100pf. I located the reverb level pot on the front panel using the resonance control (bypassed resonance function in the amp). I replaced the stock pot with a 10 k pot that I got from Peavey (thanks).

As far as reverb I can get Dick Dale levels of reverb at maximum as it is set up right now. Down side there is some noise in the amplifier that I think is due to V3 having AC filament voltage (pull V3 and the amp is quiet pull V2 still have the noise) so it looks like I need to cut and hack the PCB and run wires from the preamp board to the output for the heater supply to V3.

Word of caution the op amp negative supply is part of the bias circuit supply so if it shorts the bias supply will go positive and the output tubes will be turned on hard. I scoped and measured the bias voltage on my amp with the reverb running and saw no effect on the bias voltage. However if you cause a short by backwards polar capacitor short to chassis ect. bad things could happen.

Well found where my noise was coming from it is due to ripple on the bias voltage supply. Maybe due to the added load of the reverb amplifiers and the second divider in my amp for the 6L6 tube set. I added 1100uf across the filter caps C1 and C4 with the reverb turned down now just a slight hum with your ear to the speaker. The value of caps I used may be overkill but it is what I had on hand.

If you have ever built a jfet buffer for your guitar output where you install a jfet right on the output jack of the guitar I just realized how to apply this to the reverb tank output. I have simulated a differential jfet buffer located in the tank that should balance and lower the output impedance of the reverb tank. With this as the input to a differential amplifier the noise from the tank should be close to none. I will test this out and if it works out will post the schematic.

[StringsOfJoy]

It took me a while to digest what you were saying, but the general idea seems to be that regardless of how the additional resistance is accomplished it will increase the voltage on the stock capacitors (which is, in fact, what I'm trying to do elsewhere in the circuit to pump that additional negative bias voltage into the tubes.

Furthermore, the 63 V limit on the stock caps puts a lid on how much additional resistance I can create in the circuit before I risk those caps failing on me. So to get the kind of low idle plate dissipation out of my KT88s and run them cold I'd have to swap out the stock caps for something beefier (perhaps 100V caps?).

I'd forgotten to mention that I was conservatively treating the Penta KT88s as rated for 35 Watts instead of 42 Watts, so the idle plate dissipation percentages I cited above was based on 35 Watts.

[jim p]

I have been bothered by the fact one of the last mods I do to an amp is to add reverb and when I do I get the 60 Hz noise back that I have been beating down. So looking into where this is coming from figure it is pickup on the reverb tank output. One of the bad things is that the output signal from the tank should connected to the recovery amplifier with a twisted pair inside of a shielded cable with the shield only tied to the recovery amplifier side. But even then the output of the reverb tank is a 1mV to 5mV signal at maximum and relatively high impedance. In all the tank installations I have isolated the input leaving the ground of the reverb tank through the reverb output shield. So a quick fix for that is to isolate the output RCA connector and run a ground wire to the chassis of the reverb tank. You still have a high impedance output from the tank so I thought of the FET buffer I have made for the guitar pickups but do to the reverb output coil floating you can even do better then that.

The buffer for the reverb tank produces a differential signal with a possible gain of up to 20 depending on the JFET used for the buffer. What this requires is to install three resistors and one to two capacitors in the reverb tank or right at the RCA jack. The remaining components are part of the recovery amplifier circuit. The circuit I built used a 2N6550 in the reverb tank but they are sold by only a few distributors and darn expensive. What you want is a JFET with low noise and high transconductance which is equal to low source impedance for the 2N6550 this is 25 to 40 mmho for 40 to 25 ohms source impedance. Instead of the 2N6550 in order of best to OK is 2SK170, BF862, BF861B and J309 the two BF fets only come in SOT23 package. What is unique about the circuit due to the fact you float the coil across the source to gate connection of the JFET you can get gain at both the source and drain side of the JFET. About the parts at the reverb tank the capacitor across the reverb output coil will cause frequency peaking so if you like twangy 2nF will do that I have no capacitor across the coil I did try from 100pF to 3nf something you can play with. R13 is to reduce the gain variation from JFET to JFET for maximum gain you can make it zero ohms. R1 is to set the current at between 1mA to 2mA if the current is high increase this if low lower it. R4 the 100k resistor could be increased to possibly increase the bandwidth of the reverb output or lowered to load down the tank for a darker sound say 50k to 25k ohms. So the source side of the JFET is connected to the outer connector of the RCA jack this is the low impedance side of the JFET while the drain is connected to the center conductor.

The recovery amplifier is a differential amplifier the gain of 10 from the JFET would be doubled if this amplifier was just unity gain but the amp has a gain of 10 so total gain is 200. So the circuit to the JFET is completed by the parts at the recovery amplifier R3, R2 R12 and R11. C1 and C2 along with R9 and R10 provide a high pass filter to help keep the low frequencies out of the recovery signal. C6 across the input will limit the high frequency input to prevent hearing your local AM radio station (because some of the JFETs are RF FETs). The first op amp U1 is to provide equal gain and high input impedance for the negative input to U2. The op amp can be an NE5532 dual op amp it has good low noise numbers and will work well for this application.

To sum it up I have a little hum but the power supplies in the Peavey Windsor I added this too are not great on the low ripple side I should add regulators.

Remember if you have both the input and output of the reverb tank isolate you will need to connect a ground wire to the reverb chassis. Also if you have a painted tank like a Ruby need to break the paint where the four springs that suspend the reverb assembly are attached to complete the grounding.

I want to correct one thing above the Jfet buffer is acting as a current source at both the source and drain so the impedance at the source and drain is the same 1 k ohms in the circuit as shown.

[svenbyliner]

Hi Jim, I found this thread looking for some mod ideas for my windsor head and was not disappointed! A very thorough and interesting read.

Anyway, I recently had an idea for a relatively simple mod for my amp, which is an almost stock peavey windsor head (I changed out the 12AX7 preamp tubes to lower gain tubes.)

I've attached a picture to this post of what I'd like to do it. Simply put, I'd like to create a second preamp volume knob for the amp. This way I could set the two preamp volume knobs to different settings and switch remotely between the two.

Anyway, I'm a novice at this stuff (I've done mods but never designed one) so any help or advice is greatly appreciated.

In particular I'm wondering If I should put the second preamp volume knob on the amp itself, or if it's possible to put it on the pedal (to be used as the switch.) Also, how to best connect the pedal switch and the amp.

[jim p]

Just having one added pot in the preamp section would not be the same as having a clean and overdrive (dirty) channel if that is what you are trying to do. It would be similar to the boost that is in the amp with an adjustable level. You could do almost the same thing as adding one pot by just using a volume peddle between the amp and the guitar. If you cut the boost relay free and added two pots you could have two preamp gain settings and two volume settings to make the amp more like a two channel amp. The resonance pot is 1 Meg I think and I find the control to do little for the tone of the amplifier so it would work well as a second volume (level) control for the overdrive. Then you would need to add another pot in parallel with the preamp volume pot for the overdrive level. But one problem that may occur with this mod is popping on the speaker when switching between channels. The relay contacts may bounce or break before make and cause this to happen.

Another thing you can do with out mods to the amplifier is to use the loop in and out as a switchable volume control using the footswitch along with an input level control on the signal into the amplifier. So you turn the loop on and send a high level signal into the amplifier that you can control the volume on with a volume pot added to the effects loop same as the level pot with overdrive. When you turn the loop in/out off you input a low level signal into the amplifier for your clean sound that is controlled by the master volume on the amp.

[svenbyliner]

Yeah, the basic idea is to simulate a clean and overdrive channel.
My idea to control the volume of the "OD" channel was to simply wire a second pot onto the pedal I'm using (which has two good jacks and a good switch but a busted board, which is why I want to salvage it.)

Now, the second pot would be wired like a normal volume pedal for a guitar and would be placed into the signal path in the effects loop (thus "acting" like a master volume by coming after the preamp stage. And yes, this means the FX loop has to stay on all the time, but I've got an ab box that can go after the DIY switch pedal and act as an on/off for the rest of the FX loop)

This seems like the simplest way to control both the gain and volume from one pedal, though it will of course require a third output jack for the pedal (one jack to connect to the preamp, and one in and one out to connect to the effects loop signal chain.)

That part of the equation should be simple, I'm just unsure about how to wire the preamp gain pot so that it will be switchable.

I haven't had a chance to take the back panel off and look at it yet, but does the volume pot simply have two wires connecting to it that I need to splice a switch between?

If that's the case then I can easily hook up the pot and the two wires in question to a 4 conductor TRRS jack (with a matching jack installed on the pedal) then I can place another switch in the line before the jack, which would allow me to turn the new feature on and off from the faceplate of the amp.

I'm guessing though that the foot pedal I use will need a new switch to be able to work both the preamp volume and master volume simultaneously.
What would I use, a 4pdt?

[svenbyliner]

Do you think a noise gate will have any effect on the possible "popping" noise this mod might cause when switching between preamp controls?

If not, any thoughts on a buffer circuit?

[Muziekschuur]

Hi,

I have a Windsor Studio on the way. I allready have a ECC81 ready to put into V1 and maybe one asswell in V2. If that won't cut it I allways have a V-twin (Mesa Boogie) available to do some MIDBOOST cutting.

I allso have two 1x12 Trace Elliot openback cabs with Celestion superleads in them (16 ohm). So let's see what this will bring to the table.

I allso have two Fender M80 combo's wich I will use together with the Peavey for a WET DRY WET setup.

We'll see what that brings to the table.

Cheers,


Muziekschuur
The Netherlands

[BigBadWolf1171]

i own one of these windsor heads, the clean isnt the best, i personally like it. i run 3 -9 volt batts on my emgs and with the help of a 15 band EQ i can get decent cleans from it,just before it starts to break up a little.. i have no knowledge of the internals of an amp, like you fellas do,(im envious!).. i personally would love to get more gain from mine, but im clueless. id imagine i could change tubes but i really wouldnt know which to get..

[aurban18]

Why i can't see the pictures?